The stems of internal combustion engine valves are required to reciprocate at high speed and at high temperatures within valve guides while the engine is operating as well as being subjected to bending forces arising from improper seating and/or rocker arm side loads and other conditions of misalignment that in general render the stems susceptible to frictional wear which is even further aggravated under the current trend to increased engine speeds and operating temperatures and decreased availability of lubrication at the valve s stem.
Although the valve stems are generally protected by various methods, chromium plating has been most commonly used to achieve wear durability and improve performance of valve stem motion within the valve guide which acts as a linear bearing and therefore is itself subject to wear by reason of its ultimate contact with the valve stem.
Seizure or sticking of the valve stem in the valve guide is perhaps an even more common mode of stem-guide interference failure than an increase in clearance between the stem and the guide arising from wear for it characteristically results in the valve being seized in the open position which may be followed by combustion chamber leakage and possibly valve breakage in the event the engine piston hits the valve head while the stem is seized in the open position.
Chromium is generally an abundant element in the earth's crust and occurs in oxidated states ranging from Cr.sup.+2 (divalent) to Cr.sup.6+ (hexavalent). Heretofore, it was thought that only hexavalent chromium could provide an effective wear resistant surface for internal combustion engine valves for it bonded well to the metallic stem substrate and, even though hexavalent and trivalent chromium exhibit similar as deposited characteristics as a coating, trivalent chromium up until the present invention has not exhibited sufficient hardness or adherance of the coating to the valve stem to withstand the rigors of high speed and high temperature engine operation.
Examples of electrodepositing chromium coatings from a bath containing hexavalent chromium ions are disclosed in U.S. Pat. Nos. 3,930,527 and 4,108,770, the disclosures of which are incorporated herein by reference.
Even technical literature is replete with information concerning electroplating with hexavalent chromium ions and yet is silent on the subject of electroplating with trivalent chromium ions such as in the Article entitled "Hard Chromium Plating" on pages 29.14 through 29.16 in METALS HANDBOOK, Desk Edition published by the American Society for Metals (1985).
An example of an electroplating bath for plating a substrate with trivalent chromium for what appears to be for purposes of corrosion protection is disclosed in U.S. Reissue Pat. No. 29,749, the disclosure of which is incorporated herein by reference. Another example of electrodepositing a coating of trivalent chromium on a substrate for purposes of corrosion protection is disclosed in U.S. Reissue Pat. No. 31,508, the disclosure of which is incorporated herein by reference.
An example of simultaneously electrodepositing a coating of trivalent chromium and chromium oxide for corrosion protection is disclosed in U.S. Pat. No. 4,875,983, the disclosure of which is incorporated herein by reference.
An example of a process for depositing a hard smooth coating of trivalent chromium for both corrosion and wear resistance purposes by including a non-sulfur containing wetting agent in the electrodepositing bath is disclosed in U.S. Pat. No. 4,804,446, the disclosure of which is incorporated herein by reference. The patent however does not disclose whether such coating would provide a suitable wear resistant coating on an internal combustion engine valve and does not disclose or suggest the method of the present invention by which: (i) adhesion and hardness of an electrodeposited trivalent chromium coating is enhanced to the point where it provides an effective wear resistant coating and (2) chromium carbide compound formation may occur leading to the formation of complex chromium carbides in a chromium-chromium oxide matrix as a result of heat treatment after electrodeposition to provide a trivalent chromium coating on an internal combustion engine valve stem that surprisingly is even superior in resisting seizure than hexavalent chromium heretofor used for such purposes.
Thus, in a general sense, the present invention involves converting the engine valve industry from the historical practice of electroplating valve stems with hexavalent chromium ions to electroplating the stems with trivalent chromium ions that heretofor were unable to adhere to the stem sufficiently to withstand the rigors of engine operation and have now been found to be even superior to hexavalent chromium in resisting seizure due to exceptional hardness developed by post-plating heat treatment.
An even greater impetus to convert to electroplating valve stems with trivalent chromium ions rather than hexavalent chromium ions relates to the high toxicity characteristics of hexavalent chromium. The future of the hexavalent type of chromium plating used currently on valve stems is under significant scrutiny in view of the environmental regulations imposed by the Environmental Protection Agency. According to the American Conference of Government and Industrial Hygienists (who study the toxicity of materials for NIOSH and OSHA) hexavalent chromium and its compounds are known as carcinogens and, as yet, no evidence has been observed for the carcinogenic effects of trivalent chromium on laboratory animals unlike hexavalent chromium according to ACGIH publication, Sixth Edition (1991) entitled "DOCUMENTATION OF THRESHOLD LIMIT VALUES AND BIOLOGICAL EXPOSURE INDICES".
It has been surprisingly discovered that heat aging after electroplating metallic engine valve stems with trivalent chromium (1) improves the hardness of the plating and the bond between the plating and the stem and; (2) creates chromium carbides upon heat treatment from the co-deposition of both chromium metal and carbon atoms from the trivalent process chemistry used herein to enable the plating to serve as an effective wear surface on the stem under the rigors of engine operation and which, although not completely understood, may contribute to its surprising superiority over hexavalent chromium in resisting seizure as determined under simulated operating conditions.